1. Field of Invention
This patent specification is in the field of radiography and pertains more specifically to x-ray imaging using a digital flat panel detector.
2. Description of Related Art
Flat panel x-ray imaging devices that generate electrical signals related to local x-ray exposure have been developed in recent years. An example is discussed in U.S. Pat. No. 5,319,206, the entire disclosure of which is hereby incorporated by reference herein. An improvement involving the use of a gain layer is disclosed in U.S. Pat. No. 6,437,339, the entire disclosure of which is also incorporated by reference herein. These examples are direct conversion panels, in which x-ray photons are directly converted to electron-hole pairs and thus into electrical signals, and differ in this respect from indirect conversion panels in which x-ray photons are first converted to light and the light is then converted to electrical signals. It is believed that direct conversion panels have a number of advantages, including better spatial resolution.
Direct conversion flat panel x-ray imaging devices offer good spatial resolution and dynamic range properties and can replace x-ray film in a variety of radiographic procedures, such as, without limitation, chest x-ray imaging and mammography. However, there typically is a balance between x-ray dose and image quality. It is desirable in general to limit x-ray dose to the level that would just give the requisite image quality. Such panels typically use an amorphous Selenium (a-Se) based layer in which the incoming x-ray energy is converted to electron-hole pairs. An electric potential across the a-Se layer and a thin-film transistor array are used to derive the electrical signal representing the spatial distribution of the x-ray energy impinging on the panel.
One way to improve the conversion efficiency and thus the signal-to-noise ratio (SNR) for a given energy of x-rays impinging in a-Se conversion layers is to increase the electrical field sufficiently to create an avalanche effect, in which an x-ray photon is likely to generate multiple electron-hole pairs. See e.g. G. Pang, “Electronic portal imaging with an avalanche-multiplication-based video camera,: Med. Phys. 27 (4), 676-684 (2000). However, it is believed that an electric field of approximately 75 Volts per micrometer (75V/μm) or more in the a-Se layer is required to initiate and maintain the avalanche effect. While this is practical in a thin layer of a-Se, it becomes less so in the thickness typically used for medical imaging, which is 200-500 μm. To create a 75V/μm field in a 200 μm thick a-Se layer would require applying 15,000 Volts across the a-Se layer, and for a 500 μm layer would require 37,500 Volts. These voltages are difficult to accommodate in a medical device. Even more important, when the entire 200 μm or 500 μm a-Se layer is operating in the avalanche mode, an additional error is introduced because the number of electron-hole pairs that an x-ray photon would generate depends on the depth in the layer at which the first pair was generated. See e.g. D. Hunt, B. Lui, and J. A. Rowlands, “An Experimentally Validated Theoretical Model of Avalanche Multiplication X-ray Noise in Amorphous Selenium,” in Medical Imaging 2000: Physics of Medical Imaging, Proc. SPIE 3977. 106-116 (2000).
The earlier-cited patent (U.S. Pat. No. 6,437,339) discloses an approach in which an avalanche effect would be achieved in a thin layer of a material different from the a-Se layer that converts the x-ray energy to electron-hole pairs. This thin layer can be a gas of a solid material that has a much smaller x-ray absorption cross-section than the a-Se layer and a negligible depth-dependent gain. However, it is difficult in practice to build a suitable gas chamber or a solid material that as the requisite properties. Other efforts to improving conversion efficiency have focused on using different conversion materials, such as PbO and HgI, or using indirect conversion relying on CsI on avalanche Se, or attacking the challenge of improved SNR by providing in-pixel amplifiers, or tiled CMOS with CsI or Selenium. However, these proposals are believed to involve materials and techniques that are less understood for medical imaging than direct conversion a-Se layers, and to involve a number of direct challenges and solutions that still have to be proved.
It should also be noted that detectors with amorphous selenium vacuum deposited on thin film transistor array (TFT) using a thermal evaporation process such as the detector structure discussed in U.S. Pat. No. 5,319,206 can only be stored, transported or operated within a very narrow range of temperature. When the panel is subjected to temperatures close to the “glass transitional temperature” of selenium (i.e., 47 degrees C.), amorphous selenium gradually undergoes a phase change and slowly turns into microcrystalline selenium that has a totally different electrical property and is unsuitable for imaging use. These microcrystals normally first form near the pixel electrode and the higher conductivity of these microcrystals tend to “short circuit” adjacent pixels and therefore degrade the image quality (e.g., smearing the image). On the other temperature end, because of the difference between the thermal expansion coefficient of the glass substrate of TFT and the thermal expansion of selenium, below a certain temperature, such as 5 degree C., the selenium layer can physically be separated from the TFT substrate and opens the electrical contact between the two elements. This is called delamination and the detector is permanently damaged. Because of this, selenium coated TFT or the final detector unit must be kept above 5 degrees C., but operated below 40 degree C. and can only be transported using thermally insulated containers. Without proper care to these thermal conditions, detector panels can be damaged even being left out doors in winter time in most of the world, or can be damaged during air transportation if the panel is placed in the cargo space of a jet airplane for more than a few hours. In the past several years, many panels have been damaged this way.
Accordingly, it is believed that a need still remains to improve the conversion efficiency of flat panel x-ray imaging devices and the extension of temperature range for the storage, transportation, and operation of selenium over TFT. The invention of the present application is directed to meeting these needs.